1. Field of the Invention
This invention relates to a positioning system for positioning an object which is to be quickly moved to a desired position and finely positioned at the desired position such as a head of a random access recording device including an optical disc player and a head of printer equipment.
2. Description of the Prior Art
Optical disc players have high density recording capability and high speed random access capability. The track pitch (in the form of either spiral or concentric tracks) of an optical disc is about 1.6 .mu.m or so. In order to allow a light spot to follow such a fine track, the optical disc player needs a high resolution tracking control of is optical head. On the other hand, in order to enable high sped random access, the optical head has to be moved rapidly across the tracks in a radial direction of the disc.
It is very difficult to satisfy these two requirements in a single mechanism, and thus, the conventional optical disc player has two separate mechanisms--a tracking actuator for allowing the light spot to follow a track and a traversing actuator for moving the optical head to traverse the tracks. In order to achieve high speed address seeking, a voice-coil-type linear motor is used as the traversing actuator with a velocity sensor usually installed therein. The velocity of the traversing actuator is sensed by the velocity sensosr, and is fed back to a traversing control system. This constitutes a velocity feedback loop for stabilizing the positioning control of the optical head. An example of such an apparatus is disclosed In Japanese published patent application No. 57-181436, published Nov. 8, 1982, and entitled "Optical Disc Apparatus".
Numerical control has been used in various control systems with the progress of digital signal processing technology and semiconductor technology, but its application has been limited to the fields of control systems for plants, industrial robots and the like. An example of such a system is disclosed in U.S. patent No. 4,463,300, issued Jul. 31, 1984, and entitled "Linear Motor Digital Service Control". These systems operate in a narrow bandwidth of 10 rad/sec or so in general. On the other hand, the control system of an optical disc player usually requires a wide bandwidth of about 10 krad/sec . Digital control systems a sampling device, and its sampling frequency must be set to at least 20 times the width of the bandwidth of the controller. Therefore, the sampling period of the conventional controllers was set experimentally at about 10 msec, but the sampling period of the control system of the optical disc player should be set 0.01 msec or so. It was almost impossible to obtain a calculating speed high enough to operate such a control system by using conventional digital processors (a microprocessor, a digital signal processor and so on). Therefore, an approach to apply digital technology to the optical head control system has been neglected up to the present.
However, a control system using analog circuits cannot avoid a DC offset and thermal drift, which cause positioning errors. Further, the control system composed of analog circuits needs passive parts such as capacitors and resistors for building compensation filters for stabilizing the closed loop system. Even if the control system is mounted to a custom IC (integrated circuit), these passive parts must be mounted outside of the IC as external parts. For this reason, the control system composed of analog circuits was an obstacle for realizing low-cost and small-sized equipment.